Photodetectors in which light detection cells with avalanche photodiodes (“APDs”) operating in Geiger mode are arranged in arrays have been studied in order to achieve photodetectors with high sensitivity. These photodetectors count the number of photons that are incident thereto. The APD operating in Geiger mode is a photodiode that emits one current pulse per one photon incident thereto. An array including the light detection cells with the APDs provides a current pulse having a height proportional to the number of light detection cells receiving the photons. Thus, the number of photons incident to each photodetector can be measured from the pulse height.
Radiation detectors including scintillators and photodetectors are also known. The scintillators receiving radiation rays emit visible light rays, which may be detected by photodetectors such as photodiodes and photomultipliers. The number of photons in the visible light rays emitted from a scintillator is proportional to the radiation energy incident to the scintillator. Therefore, the radiation energy may be measured by counting the number of photons included in the visible light rays emitted from the scintillator. This can be performed by means of a photodetector in which light detection cells including APDs operating in Geiger mode are arranged in an array.
Photodetector arrays are used for image sensors. Radiation detector arrays in which radiation detectors with scintillators and photodetectors are arranged in an array are included in computed tomography apparatuses using X-rays. In both the cases, many pixels are required to be disposed with a high density in order to obtain high-quality images. In these cases, wires for taking electric signals from a photodetector with many pixels arrange at a high density are generally connected using a through-electrode called “TSV (Through Silicon Via)” since it is difficult to connect these wires by wire bonding.
The TSV formed in a photodetector in which light detection cells including APDs operating in Geiger mode are arranged in an array is effective to obtain images with a high sensitivity and a high quality.
A problem is known for the arrays of light detection cells including APDs operating in Geiger mode that light rays in a wavelength band from the visible light to the far-infrared light are radiated from a light detection cell to which photons enter, and the radiation rays enter adjacent light detection cells (“optical crosstalk”).
A technique is known to solve this problem, in which trenches are formed between adjacent two light detection cells with APDs from a side opposite to the substrate on which the light detection cells are formed, i.e., from the light detection cell side, and filled with black resist or a metal such as tungsten. This blocks light rays from adjacent light detection cells to prevent the optical crosstalk.
The optical crosstalk prevention technique has the following problems. During the manufacture, the light detection cells with the APDs are generally subjected to a high-temperature process performed at a temperature of 400° C. or more. Therefore, black resist, which generally does not bear such high temperatures, cannot be actually used.
A metal such as tungsten is generally filled into the trenches by sputtering or chemical vapor deposition (CVD). However, these methods, which are generally used to form thin films, are not capable of filling the trenches efficiently. Therefore, voids may be caused to reduce the reliability.